Semiconductor process

ABSTRACT

A process by which photonic band gap structures are created by etching trenches in indium gallium arsenide, preferentially infilling the trenches with indium phosphide and forming multiple layers of interleaved regions of indium gallium arsenide and indium phosphide.

This application is the U.S. national phase of international applicationPCT/GB00/03788 filed Oct. 3, 2000 which designated the U.S.

FIELD OF THE INVENTION

This invention relates to a method of processing semiconductor materialsin order to create photonic band gap materials.

Semiconductor materials have an electrical band gap, which existsbetween the conduction band and the valence band, which electrons maynot occupy due to an absence of energy levels. In a similar manner,there are some materials that exhibit a photonic band gap in which lightof a given wavelength may not propagate. Such photonic band gapmaterials have great potential for use in constraining and trappinglight for example in waveguides, optical memory, cavities of lightemitting devices etc.

BACKGROUND OF THE INVENTION

A number of methods of making photonic band gap crystals have beenproposed; Lin et al, “A three-dimensional photonic crystal operating atinfrared wavelengths”, Nature, vol. 394, Jul. 16, 1998, pp 251-253,report a ‘woodpile’ crystal of polycrystalline silicon rods which wasfabricated by depositing a layer of silica, masking the silica in adesired pattern, etching the unmasked silica and filling the trencheswith polycrystalline silicon. The surface of the wafer was then madeflat using chemical mechanical polishing and the process was repeated,with successive layers of polycrystalline silicon rods being formed inalternating orthogonal orientations. Once a sufficient number of layershad been deposited, the SiO₂ was removed in an HF/H₂O solution. Thepolycrystalline silicon rods had a thickness of 1.6 μm, a width of 1.2μm and the spacing between rods was 4.2 μm. The photonic band gapcrystal had a stop band of 10-14.5 μm with an attenuation ofapproximately 12 dB per cell.

Alternative techniques used to fabricate photonic band gap materialsinclude the use of two photon excitation resins (Cumpston et al,“Two-photon polymerisation initiators for three-dimensional optical datastorage and microfabrication”, Nature, volume 398, Mar. 4, 1999, pp51-54) and radio frequency bias sputtering of Si/SiO₂ (Hanaizumi et al,“Propagation of light beams along line defects formed in a Si/SiO₂three-dimensional photonic crystals: Fabrication and observation”,Applied Physics Letters, volume 74, number 6, Feb. 8, 1999, pp.777-779). For a more general description of photonic band gap materialsand crystals see J D Joannopoulos, R D Meade, J N Winn, “Photoniccrystals: Molding the Flow of Light”, Princeton University Press, ISBN0-691-037447-2.

The disadvantage with these known techniques is that they are notcompatible with fabrication techniques for opto-electronic devices,which will reduce the level of integration that will be possible.Additionally, techniques which rely on mechanical steps, such aspolishing, will have problems when attempting to fabricate crystalswhich require physical dimensions having a very small resolution, forexample sub-micron resolution.

SUMMARY OF THE INVENTION

According to a First aspect of the present invention there is provided amethod of making a photonic band gap material, the method comprising thesteps of;

(a) growing an epitaxial layer of a first semiconductor material onto asubstrate;

(b) applying a mask to selected areas of the first semiconductormaterial and etching away the non-masked areas of the firstsemiconductor material to form a plurality of recesses;

(c) selectively growing an epitaxial layer of a second semiconductormaterial to fill the plurality of recesses created by the etching of thefirst semiconductor material; characterised in that the method comprisesthe further steps of;

(d) growing a further epitaxial layer of the first semiconductormaterial over the first semiconductor material and the secondsemiconductor material;

(e) applying a mask to selected areas of the further epitaxial layer ofthe first semiconductor material and etching away the non-masked areasof the further epitaxial layer of the first semiconductor material toform a further plurality of recesses, said further plurality of recessesbeing rotationally displaced with regard to the plurality of recessesformed within the preceding layer of the first semiconductor material;

(f) selectively growing a further plurality of epitaxial layers of thesecond semiconductor material to fill the recesses created by theetching of the first semiconductor material; and

(g) repeating steps (d), (e), and (f) as required to form asemiconductor product having a plurality of layers of interleavedregions of the first semiconductor material and the second semiconductormaterial, the regions in each of the layers being rotationally displacedwith regard to the regions in the adjacent layers.

The advantage of this method is that the deposition and etchingprocesses allow for very accurate control of the recesses and layers,giving significantly increased control over the dimensions of thestructure. This enables semiconductor structures having a higher qualityto be made. The deposition and etching processes are the same as thoseused in the fabrication of other semiconductor devices, enablingstructures made according to the above to be readily integrated withother semiconductor devices.

Preferably the method is further characterised by the etching of thesemiconductor product to selectively remove substantially all of thefirst semiconductor material whilst leaving the second semiconductormaterial substantially unaffected. Alternatively, the method is furthercharacterised by the etching of the semiconductor product to selectivelyremove substantially all of the second semiconductor material whilstleaving the first semiconductor material substantially unaffected. Theadvantage of this is the increased difference in permitivity which isgained by removing one of the semiconductor materials will enhance theproperties of the semiconductor product. The adjacent layers ofsemiconductor materials may be rotationally displaced by substantially90°.

Preferably the first semiconductor material is indium gallium arsenideand the second semiconductor material is preferably indium phosphide.This enables the semiconductor structure to be made using well knownsemiconductor fabrication processes. Additionally, this has theadvantage that the semiconductor structure can function in one of thetelecommunications wavelength windows and that such structures can beintegrated within opto-electronic components that operate within one ofthe telecommunications wavelength windows.

Preferably the indium phosphide is selectively grown in the presence ofa chloride compound which gives the advantageous deposition of theindium phosphide is selectively grown in the presence of phosphorustrichloride (PCI₃)

According to a second aspect of the present invention there is provideda photonic band gap material fabricated using a method as describedabove.

According to a third aspect of the present invention there is providedan opto-electronic device comprising a photonic band gap materialfabricated as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the following figures in which;

FIG. 1 shows a schematic depiction of a number of different photoniccrystal structures; and

FIG. 2 shows a schematic depiction of a process according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a number of different photonic crystal structures. FIG. 1ashows a one-dimensional photonic crystal, FIG. 1b shows atwo-dimensional photonic crystal and FIG. 1c shows a three-dimensionalphotonic crystal. All of these photonic band gap crystals are formedfrom two different materials, with the properties of those materials andthe geometry of the crystals affecting the properties of the photoniccrystals. The efficiency of a photonic crystal, i.e. the volume ofcrystal required to provide a given photonic effect, is dependent uponthe difference in permitivity between the two materials which form thecrystal and thus the greatest efficiency is obtained when one of thematerials is not present, i.e. the crystal is formed from one material.Three-dimensional photonic crystal structures which are of greatinterest and which are referred to as ‘woodpile’ structures consist ofrods of a crystal material arranged in successively orthogonal layers.

Although the strict definition of a photonic band gap material is one inwhich certain wavelengths may not propagate it is to be understood thatin the following discussion reference to photonic band gap materials andcrystals also includes those materials and crystals which exhibit somedegree, or substantially all, of the properties associated with photonicband gap materials and crystals.

FIG. 2 shows the process of the present invention being used tofabricate a ‘woodpile’ three-dimensional photonic crystal. A number ofepitaxial layers of indium gallium arsenide (InGaAs) 20 are grown to adesired thickness (see FIG. 2a), for example 1 μm, on an indiumphosphide (InP) substrate 10 in a MOVPE (Metallorganic Vapour PhaseEpitaxy) reactor. The InGaAs may be grown at a temperature of 660 C witha reactor pressure of 810.3 torr. The Group III compounds used aretrimethylindium (which is stored at 35.23C) and trimethylgallium (whichis stored at −12.03C). The group V compound is arsine (AsH₃) which issupplied in a 5%:95% arsine/hydrogen mix. A 0.2 μm thick silicon dioxide(SiO₂) layer 30 is then deposited upon the InGaAs surface. This layer isthen covered with a photoresist 40, for example Shipley Microposit S1805which is applied by spinning at approximately 5000 rpm. The surface isthen exposed to a u-v source through a chrome mask having the form of anumber of parallel stripes each, in this exemplary case, being 1 μm wideand having a spacing of 1 μm between each adjacent stripe. The u-vexposed photoresist is then removed using a developer, for exampleShipley Developer MF 319 and any further excess photoresist is thenremoved by exposure to oxygen plasma for approximately 45 minutes at500W.

The SiO₂ surfaces which have been exposed by the removal of thephotoresist are then etched, preferably using a 50:50 mix oftrifluoromethane (CHF₃) and hexafluroroethene (C₂F₆) at a pressure of160 mTorr, a temperature of 14° C. and a power of 175W. The etch isapplied three times for a period of 5 minutes each. It has been foundthat exposure for periods longer than 5 minutes can cause thephotoresist to breakdown. An etching process is then performed in aknown manner to remove the InGaAs material not covered by the stripedSiO₂ layer, thus forming a plurality of parallel trenches 50 (see FIG.2b) located between the regions of the silicon dioxide. The etchingprocess is controlled so that the InGaAs material is removed down to theInP substrate, i.e. in this case, forming a trench having a squarecross-section with a dimension of 1 μm. A preferred etching process is adry etch in a mixture of hydrogen and methane, with a power of 200W, aDC bias voltage of approximately 470V, a pressure of 70 mTorr and at atemperature of approximately 13° C.

Indium phosphide (InP) is then grown in the presence of phosphorustrichloride (PCI₃), in a known manner (see M J Harlow et al, “Theinfluence of PCI₃ on planarisation and selectivity of InP regrowth byatmospheric MOVPE”, 7th International Conference on Indium Phosphide andRelated Materials, Sapporo, Japan pp 329-332 & SD Perrin et al“Planarised InP regrowths around tall and narrow mesas usingchloride-MOVPE” 11th International Conference on Indium Phosphide andRelated Materials, Davos, Switzerland, pp 63-66), which causeslinearised planar growth of InP within the trenches formed by theetching of the InGaAs material. As described by Harlow et al op cit, theInP is selectively grown only within the trenches, and not upon the SiO₂masked-lnGaAs mesas which define the trenches. The InP/PCI₃ growthprocess continues until the trenches are completely filled with InP.Following the growth of InP into the trenches, the SiO₂ mask is removedby wet etching with hydrofluoric acid (HF) for 2 minutes. FIG. 2c showsa perspective view of an InP substrate 10 and alternating regions of InP60 and InGaAs 20.

Subsequently, a further plurality of epitaxial layers of InGaAs materialare grown over the entire surface area of the composite InGaAs/InPmaterial. The InGaAs is deposited using the conditions described above,except that the temperature is ramped up to 660° C. in an atmosphere ofphosphine (PH₃) to prevent desorption of the phosphorous. A layer ofsilica is deposited over the InGaAs, followed by a layer of photoresistboth as described above. For the second, and subsequent, resist layers athicker layer is required (as is known), for example Shipley MicropositS1813 which is also spun on at approximately 5000 rpm. The photoresistis exposed to a u-v source through a chrome mask identical to the onedescribed above, but at an orientation of 90° to that of the first mask.The u-v exposed photoresist is then removed using a developer and anyfurther excess photoresist is then removed by exposure to oxygen plasma,as described above. The exposed SiO2 regions are then etched using theCHF₃/C₂F₆ mixture described above and then the InGaAs is etched,creating a plurality of trenches that are orthogonal to the trenches nowfilled with InP.

Again, the etching process is controlled to produce trenches of adesired depth and so that the InGaAs material in the layer below themasked layer is not effected. A further InP/PCI₃ growth process (asdescribed above) is performed in order to fill these trenches withplanar InP. The SiO₂ mask regions are then stripped off the InGaAsregions of the uppermost layer. The processes of depositing InGaAs, SiO₂and photoresist and subsequently etching to form trenches and thenselectively growing InP to fill the trenches are repeated until adesired number of layers have been formed of the composite InGaAs/InPstructure. FIG. 2d shows a structure having an InP base substrate 10 andthree layers of alternating regions of InGaAs 20 and InP 60.

Once sufficient layers have been formed, say, for example, eight layers,the structure will exhibit photonic band gap material properties, butthese properties will be enhanced by removing one of the materials fromthe structure as this increases the difference in permitivity betweenthe two materials of the structure. This can be achieved by the removalof either the InGaAs or the InP using a suitable etchant or solvent. Asthe structure has been deposited upon an InP substrate it is preferredto remove the InGaAs. The InGaAs is preferably removed by applying 70%concentrated nitric acid (HNO₃) to the structure for 2 hours. If the InPis to be removed, it may be preferred to deposit a buffer layer over theInP substrate, or alternatively to use a different substrate material.

Although the above process has described the use of ultra-violetlithography with a chrome mask, it should be understood that this isonly one possible method of patterning the silicon dioxide layer andthat other methods which allow similar patterns to be formed on thesilicon dioxide layer are equally suitable for use with the presentinvention. If greater dimensional resolution is required, for examplesub-micron resolution, then a direct write lithography process should beused, for example electron-beam lithography. This does not preclude thefuture use of more advanced lithographic processes that have beenproposed, for example X-ray lithography, to achieve sub-micronresolution.

Additionally, only the use of indium gallium arsenide (InGaAs) withindium phosphide (InP) has been discussed, but it is to be understoodthat other materials could be used in place of InGaAs as long as it ispossible to lattice match the material with indium phosphide.Additionally, InP may be replaced by another material that exhibits theplanar regrowth that InP does when deposited in the presence of PCI₃ (oranother halogen compound, Harlow et al, op cit.). However, the use ofInP is preferred as it is commonly used in the fabrication of opticaland electro-optical components in telecommunications equipment and canemit light in the telecommunications transmission windows centred around1.3 μm and 1.55 μm

The size of the rods in the woodpile structure determines theproperties, for example the wavelength(s) of the stop band of thephotonic band gap material. Thus these properties are determined by thewidth of the mask details (which determines the ‘width’ of the rods) andthe control of the depth of the material deposited (which determines the‘height’ of the rods and using MOVPE techniques this can be controlledto give sub-micron resolution). For an InP woodpile band gap materialmade using the method of the present invention and having a rodcross-section of 1 μm×1 μm it is believed that the stop band wavelengthwill be in the region of 10 μm. To fabricate a band gap material havinga stop band in the region of the telecommunications transmissionwavelengths (centred around 1.3 μm and 1.55 μm) it is believed that asub-micron rod dimension will be required.

Using the method of the present invention it is possible to fabricaterods having a substantially square cross-section or a rectangularcross-section and the relative magnitude of the height and the width ofthe rods can be varied to vary the property of the photonic band gapmaterial (although as the dimensions of the rod are decreased, thewavelengths of the stop band of the photonic band gap material will bedecreased). It will be clearly understood that the structure describedabove having rods with dimensions of 1 μm×1 μm is given solely as anexample. It will also be understood that the ratio of rod size to thespace separating adjacent rods (referred to as the mark-space ratio) isalso given solely by way of example and may be varied to alter theproperties of the structure.

Although the examples described above show that the layers of rods inthe crystal are arranged at right-angles to the preceding layer, it isto be understood that this is merely one option and that any rotationaloffset from the preceding layer can be chosen, although this will havean effect on the properties of the crystal being fabricated. Forexample, if the offset angle is 60 degrees then the rods of the first,fourth, seventh and every subsequent third layer will be parallel (aswill the rods of the second, fifth and eighth layers and the rods of thethird, sixth and ninth layers, etc.).

Also, it is also possible to introduce a lateral offset into thestructure of the crystal so that for layers in the same orientation therods are not vertically aligned i.e. for a crystal with a 90 degreeoffset between subsequent layers, each layer is offset by 0.5 μm fromthe preceding parallel layer such that the first, third and fifth layersare all parallel to each other whilst the first and fifth layers arevertically aligned [as are the third and the seventh layers] (for anexample of such a crystal structure see Lin et al, “A three-dimensionalphotonic crystal operating at infrared wavelengths”, Nature, vol. 394,Jul. 16, 1998, pp 251-253).

Photonic band gap material and structures as described above may beincorporated into opto-electronic devices, in particular all-opticaldevices, as, for example, reflectors, mirrors, confinement structures,waveguides, etc.

What is claimed is:
 1. A method of making a photonic band gap material,the method comprising: (a) growing an epitaxial layer of a firstsemiconductor material onto a substrate; (b) applying a mask to selectedareas of the first semiconductor material and etching away thenon-masked areas of the first semiconductor material to form a pluralityof recesses; (c) selectively growing an epitaxial layer of a secondsemiconductor material, while the mask applied in step (b) is still inplace, to fill the plurality of recesses created by the etching of thefirst semiconductor material; (d) removing the mask and subsequentlygrowing a further epitaxial layer of the first semiconductor materialover the first semiconductor material and the second semiconductormaterial; (e) applying a mask to selected areas of the further epitaxiallayer of the first semiconductor material and etching away thenon-masked areas of the further epitaxial layer of the firstsemiconductor material to form a further plurality of recesses, saidfurther plurality of recesses being rotationally displaced with respectto the plurality of recesses formed within the preceding layer of thefirst semiconductor material; (f) selectively growing a epitaxial layerof the second semiconductor material, to fill the recesses created bythe etching of the first semiconductor material; and (g) repeating steps(d), (e), and (f) as required to form a semiconductor product having aplurality of layers of interleaved regions of the first semiconductormaterial and the second semiconductor material, the regions in each ofthe layers being rotationally displaced with respect to the regions innthe adjacent layers for producing photonic band gap material.
 2. Amethod of processing semiconductor materials according to claim 1, themethod being further characterised by the etching of the semiconductorproduct to selectively remove substantially all of the firstsemiconductor material whilst leaving the second semiconductor materialsubstantially unaffected.
 3. A method of processing semiconductormaterials according to claim 1, the method being further characterisedby the etching of the semiconductor product to selectively removesubstantially all of the second semiconductor material whilst leavingthe first semiconductor material substantially unaffected.
 4. A methodof processing semiconductor materials according to claim 1, whereinadjacent layers of semiconductor materials are rotationally displaced bysubstantially 90°.
 5. A method of processing semiconductor materialsaccording to claim 1, wherein the first semiconductor material is indiumgallium arsenide.
 6. A method of processing semiconductor materialsaccording to claim 1, wherein the second semiconductor material isindium phosphide.
 7. A method of processing semiconductor materialsaccording to claim 6, wherein the indium phosphide is selectively grownin the presence of a chloride compound.
 8. A method of processingsemiconductor materials according to claim 7, wherein the indiumphosphide is selectively grown in the presence of phosphorus trichloride(PCl₃).